An explosion has rocked a school in Addis Ababa, injuring at least 28 students, three seriously.

The blast occurred in a packed classroom on Monday morning, at Kokebe Tsebha Elementary School, attended by 4,500 children.

When I turned round, the room was full of smoke and there was glass everywhere

Abebe Kidane Teacher

The 4th grade students were rushed to the nearest hospital - most are believed to be about 12 years old.

Police officials said that a schoolboy had brought a grenade into the school, and unknowingly detonated it.

As I arrived at the scene, school teachers and students gathered outside the school compound to recount stories of what had occurred this morning.

Blackboard

According to teachers, there were 59 children in the classroom when the grenade went off at 1030 am local time (0730 GMT).

Abebe Kidane was teaching at the time.

"I was writing on the blackboard when the explosion when off. When I turned round, the room was full of smoke and there was glass everywhere. The children were screaming and crying," he said.

"In the panic, I tried to drag the injured children out of the room. Other children rushed out and even one student jumped over the veranda on the second floor of the building - in order to get away from the blast."

The remnants of what occurred this morning could still be seen when I arrived on the scene.

The smell of smoke was still evident - even before entering the classroom.

Trails of blood led from the classroom to the outside of the building.

Shattered

Inside, textbooks and exercise books spattered in blood still lay open, as students had rushed out of the classroom in a state of panic.

The walls, floor and even the blackboard where a teacher was giving a maths lesson at the time, were covered in blood.

Windows on both sides of the small classroom, were shattered.

Police investigators arrived quickly on the scene and local people living around the school in Addis Ababa's eastern Kebena area - offered their cars to transport the injured children to the nearest hospital.

Addis Ababa police¿s forensic department and investigators were examining and photographing the classroom in order to establish exactly what happened.

According to officials, it was a random incident by an 11-year-old student.

The proliferation of small arms and light weapons coming into Ethiopia from neighbouring countries as well as and arms left over from previous civil wars is a serious concern for the government.

Illegal weapons such as pistols are easily available and grenades and unexploded ordinances are sometimes found lying around by children.